In hot rolling of strip material, the thermal camber and wear of the work rolls, as well as elastic deformations within a rolling program are subject to relatively large changes. Without correction by final control elements, the cambering of the work rolls increases continuously with increasing rolled material throughput. Because of changing thermal camber, the roll contour increasingly deviates from the nominal contour, for instance, a parabola.
When rolling one single width, a plurality of strips is rolled consecutively at the same (or approximately the same) width within a single rolling program. Rolling at a single width affects not only the magnitude of the strip profile predetermined for a very specific point (for instance C.sub.40 or C.sub.25), but also the strip profile shape overall. The definition of the strip profile herein, for a very specific point, is the difference between the thickness of the strip at its center and the average value of the thickness measured on each side as to be spaced from the strip edge at the point C.sub.40, corresponding to a spacing of 40 mm.
The increasing fall-off of the thermal cambering of the rolls leads to considerable profile anomalies in the strip in the region near the edge. This is meant to include all deviations of the strip from the ideal (for instance parabolic) course of the strip profile. Both thickening in the edge region (beads, edge build-up) and reduction of the thickness in the edge region are types of profile anomalies to be avoided in actual rolling practice. Such profile anomalies greatly restrict the rollable length in one specific width. The roll lengths in one single width is defined as the sum of all strip lengths which are rolled at one single width or approximately at the same single width.
Compensating for the change of the thermal crowns and the work roll wear by suitable control members such as displacement and/or bending members, for instance, "CVC" (Continuously Variable Crown) displacement (see, e.g., DE 30 38 865 C1) or by a suitable cooling means.
EP 0 276 743 B1 teaches adjusting the horizontal displacement of the work rolls and the bending forces acting upon these work rolls in a group of rolling stands located on the upstream side of a tandem rolling mill, in proportion to the rolling conditions, including the width of the strip, for controlling the cambering and/or the edge reduction of the strip. For controlling the wear and the thermal cambering of the work roll to avoid undesirable profile shapes when rolling at a single width, the work rolls in a group of rolling stands, located at the downstream side, are moved to and from at predetermined intervals, irrespective of the width of the strip. The rearward stands herein are displaced by a specific amount in the opposite direction after every strip. If the amount of displacement has attained a maximum value, the displacement direction is reversed. The wear of the work rolls is made more uniform across a larger region because of this cyclic displacement.
EP 0 219 844 B1 discloses determining the profile of every work roll in axial direction, which changes during the time interval between a change of the work rolls. Thereupon, the configuration of the gap between the top and bottom work roll, on the basis of the determined roll profile, is fixed in axial direction as a function of the magnitude of a relative displacement of the roll positions, in order to determine that magnitude of the displacement of the roll positions which establishes, as flat as possible, a configuration in axial direction for the gap within the contact region between the rolled strip and the work rolls. Thus, in this case, the smoothing of the rolling gap is desired.
The known measures are, however, inadequate to fulfill increased requirements as far as the profile accuracy and the surface evenness under extreme marginal conditions is concerned. These today consist of the fabrication of hot strips being able to establish the rolling programs in a flexible manner. Apart from larger thicknesses and material changes, width changes in direction narrow and wide are desired (mixed rolling). In addition, the quantity of the strips of the same width within one rolling program is to be increased.
It is therefore an object of the invention to provide a method and an apparatus by means of which the requirements of profile accuracy and surface evenness of the rolled strip can be fulfilled, in spite of flexible rolling programs.
Another object of the invention is to provide a method and an apparatus in which profile anomalies such as the thickening and the reduction of thickness in the edge regions of the strips are avoided.